* Include puppet version requirements in metadata

* Support Ruby 2.3.1

This patch is needed to support the new Puppet v4 AIO which uses Ruby
2.3.1.

Fix params validation + specs

* Move params validation to corresponding class: We only need to valide the
params when they need to be set / are actually used. Since Puppet 4.6 or 4.7,
stdlib's validate_string function doesn't accept undef anymore, so this has
to be changed.

* Remove specific error checking in specs: types checking validate_* functions
have different error messages depending on the version the stdlib module, the
version of puppet, maybe the version of ruby, the phase of the Moon... who knows.

Fix map calling in hash

Fix the path of the map database

map defines the default path by prepending '/etc/postfix/' to $name, so
this doesn't work when the name is already the absolute path. We need
to force the path passed to map.

The postmap command needs the complete path of the database

* Add a map define to create postfix maps

It is similar to existing hash define, but this define can create maps
of any type, not just hash.

* Refactor postfix::hash to avoid duplicate code

postfix::hash uses now the new postfix::map to create the hash

* Ensure exim4 is stopped on Debian before testing
* Ensure sendmail ist stopped on RedHat systems before testing
* Support Ubuntu 16.04 instead of 12.04

Add conffile define

* Add $manage_mailx boolean to control whether mailx is managed (replacement for PR #141)

* add more docs for $manage_mailx

* fix scope of variable

May address #130

set mode 0644 for /etc/mailname

Add manage_root_alias parameter

consistent formating of the documentation

add ensure class arguments for the packages